This invention generally relates to electromagnetic clutches and, more particularly, to an improved magnetic rotatable member for an electromagnetic clutch.
Electromagnetic clutches generally are mounted on a device having a drive shaft to selectively drive the device from an external power source. A conventional electromagnetic clutch is described, for example, in U.S. Pat. Nos. 3,044,594 and 3,082,933. Such a conventional electromagnetic clutch is shown in FIG. 1 and includes magnetic pulley 3 rotatably mounted on tubular extension 1a of device 1 through bearing 2. Magnetic pulley 3 is rotated by an external power source through a belt (not shown). Annular electromagnetic device 4 is disposed in a stationary position in a hollow portion defined by pulley 3 and is secured on device 1 through support plate 4a. Hub 6 is secured on outer terminal end of drive shaft 5 and armature plate 8 is supported by a plurality of leaf springs 7 on and around hub 6 at a predetermined radial gap. Armature plate 8, which faces the axial end surface of pulley 3 at a predetermined axial gap, is capable of limited axial movement.
In the above described electromagnetic clutch, pulley 3 includes outer cylindrical portion 31 on which belt-receiving groove 31a is formed, inner cylindrical portion 32 and axial end plate portion 33 connecting the outer and inner cylindrical portions 31 and 32. Axial end plate portion 33 has a plurality of concentric slits 33a, 33b which form concentric annular magnetic pole faces at an axial end thereof. Armature plate 8 also is provided with slit 8a on the surface opposite the concentric slits of pulley 3.
In this conventional electromagnetic clutch, pulley 3 is formed as a single body of magnetic material, such as steel, by a two-step process including a forging step followed by a machining step. In the forging step, U-shaped annular ring A' is formed as shown in FIG. 2a, and in the machining step, ring A' is machined to the accurate diameter of pulley 3, i.e., the oblique line portion shown in FIG. 2b is cut by machine. Concentric slits 33a, 33b then are formed by a press punching step. Thus, as described above, the pulley is formed in several steps, such as the forging and machining steps, which waste much material as evident from a comparison of FIG. 2a and FIG. 2b. This waste of material increases the cost of the magnetic clutch.
In addition, if it is desired to increase the rotation speed of the pulley, the diameter of pulley 3 should be reduced, which means the distance between inner diameter D.sub.4 of outer cylindrical portion 31 and outer diameter D.sub.5 of inner cylindrical portion 32 should be reduced. As a result of such a reduction, however, the distance between concentric slits 33a, 33b is reduced, which in turn reduces the frictional contact surface between the armature plate and the pulley. This has the additional disadvantage of reducing the transmitted torque.
To resolve the above described disadvantage of reduced transmission torque, the frictional contact surface between axial end plate portion 33 and armature plate 8 can be increased. For example, outer diameter D.sub.2 of axial end plate portion 33 and outer diameter D.sub.1 of armature plate 8 can be increased. However, if these diameters are increased, the amount of wasted material to form the final shape of the pulley is increased, which again increases the cost of the magnetic clutch.